bad diagnostic when macro tries to use stmt as expr

[durka]

Given this macro:

macro_rules! foo {
    ($s:stmt) => { $s }
}

When called in expression context, for example println!("{}", foo!(42)), the error message refers to "unexpected token 42;", thereby conjuring a semicolon out of the ether. We aren't JS so we shouldn't conjure semicolons out of the ether :)

[GuillaumeGomez]

So if we follow the trace;
https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/parser.rs#L398
https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/parser.rs#L387
https://github.com/rust-lang/rust/blob/master/src/libsyntax/print/pprust.rs#L249

I think the "error" comes from these lines.

[durka]

@GuillaumeGomez Hmm, I guess it must be that the "statement" 42 ends up as essentially Literal(Integer(42), Some(intern(";")))?

[Mark-Simulacrum]

So we still produce a semicolon, but the error is much better (though still far from ideal):

error: expected expression, found `42;`
 --> test.rs:2:20
  |
2 |     ($s:stmt) => { $s }
  |                    ^^ expected expression
...
6 |     println!("{}", foo!(42));
  |                    -------- in this macro invocation

[estebank]

The current output provides an incorrect suggestion:

error: expected expression, found `42;`
 --> src/main.rs:2:20
  |
2 |     ($s:stmt) => { $s }
  |                    ^^ expected expression
...
6 |     println!("{}", foo!(42));
  |                    -------- in this macro invocation
  |
  = note: the macro call doesn't expand to an expression, but it can expand to a statement
  = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: add `;` to interpret the expansion as a statement
  |
6 |     println!("{}", foo!(42););
  |                            +

[estebank]

Triage: no change.

[estebank]

Triage: no change.

[RodBurman]

Building the foo macro example.

macro_rules! foo {
    ($s:stmt) => { $s }
}

fn main() {
    println!("{}", foo!(42));
}

With the current nightly version:

cargo 1.100.0-nightly (b2e9d5f9d 2026-09-02)
release: 1.100.0-nightly
commit-hash: b2e9d5f9db3fb1c454ab84f10c16508984a266e2
commit-date: 2026-09-02
host: aarch64-apple-darwin
libgit2: 1.9.6 (sys:0.21.0 vendored)
libcurl: 8.7.1 (sys:0.4.90+curl-8.21.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 3.6.3 9 Jun 2026
os: Mac OS 26.6.2 [64-bit]

Gives the following output:

   Compiling baddiagmac v0.1.0 (/Users/rod/code/rust/triage/2026/09/baddiagmac)
error: expected expression, found `stmt` metavariable
 --> src/main.rs:2:20
  |
2 |     ($s:stmt) => { $s }
  |                    ^^ expected expression
...
6 |     println!("{}", foo!(42));
  |                    -------- in this macro invocation
  |
  = note: the macro call doesn't expand to an expression, but it can expand to a statement
  = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: add `;` to interpret the expansion as a statement
  |
6 |     println!("{}", foo!(42););
  |                            +

error: could not compile `baddiagmac` (bin "baddiagmac") due to 1 previous error

So the semi-colon from nowhere is cured. But the incorrect/unhelpful help message to add a semi-colon still exists.